The present invention relates to improved carbon blacks and a process for producing them.
Carbon blacks are used extensively as reinforcing blacks in rubber mixtures for the tire industry. The characteristics of the carbon black, combined with the properties of the rubber compositions used, influence the performance properties of the finished tires.
Requirements for the properties of the rubber compositions of the tires include high resistance to abrasion, the lowest possible rolling resistance, and the best possible wet skid behavior. Both of the latter properties are significantly affected by the viscoelastic behavior of the tread composition. For periodic deformation, the viscoelastic behavior can be described by the mechanical loss factor tan .delta. and, in the case of stretching or compression, by the dynamic modulus of extension .vertline.E.sup.* .vertline.. Both quantities depend strongly on the temperature. The wet skid behavior of the tread composition is generally correlated with the loss factor at about 0.degree. C., tan .delta..sub.0, while the rolling resistance is correlated with the loss factor at about 60.degree. C., tan .delta..sub.60. The higher the loss factor at the low temperature, the better, generally, is the wet skid behavior of the tire composition. On the other hand, the lowest possible loss factor at the higher temperature is required to reduce the rolling resistance.
The abrasion resistance and the viscoelastic properties, and thus also the loss factor of the tread mixture, are significantly determined by the properties of the reinforcing black used in the rubber formulation for the tire. The important factor here is the specific surface area, and particularly the CTAB (cetyltrimethylammonium bromide) specific surface area which is a measure of the portion of the carbon black surface that is effective in rubber. The abrasion resistance and tan .delta. increase with increasing CTAB surface area. The CTAB designation is well known in this industry. The DBP (dibutyl phthalate) absorption and the 24M4 DBP absorption are other important carbon black properties of the initial structure or the residual structure following mechanical loading of the black. The DBP and 24M4 DBP designations are well understood in this field of technology.
Carbon blacks having CTAB surfaces between 20 and 190 m.sup.2 /g and 24M4 absorption numbers between 60 and 140 ml/100 g are suitable for tire rubber compositions.
The mean particle size is used to classify the carbon black according to ASTM (American Society for Testing and Materials) D-1765. This involves a four-character alphanumeric nomenclature, in which the first letter (either N or S) makes a statement about the vulcanization properties, and the first digit of the following three-digit number indicates the mean particle size. This ASTM classification is admittedly quite coarse. There can be substantial variation of the viscoelastic properties of tread mixtures within a single ASTM classification range.
It has been shown that ordinary carbon black cannot affect the temperature dependence of the loss factor, tan .delta., enough so that the tread mixture will have a lower rolling resistance with equal or better wet skid behavior. The desired reduction of the rolling resistance is generally directly coupled with deterioration of the wet skid behavior. Carbon blacks which produce a low rolling resistance are called "low hysteresis" blacks.
Thus, it is desirable to use carbon blacks which, after incorporation into a rubber composition, give that rubber composition a low value of tan .delta..sub.60 with an equal or greater value of tan .delta..sub.0, with equal or comparable analytical properties for the carbon black, especially equal CTAB surface. The ratio of the two values, tan .delta..sub.0 /tan .delta..sub.60 should be greater than for ordinary carbon blacks with the same CTAB surface.
DE 43 08 488 corresponding to U.S. Pat. No. 5,639,817 discloses carbon-black-reinforced rubber compositions which are supposed to have a "steeper" tan .delta./temperature curve. This statement refers to the comparison of two rubber mixtures, the first made with a carbon black according to the invention of DE 43 08 488 and the second made using a standard carbon black N339. However, the standard N339 carbon black has twice as great a CTAB surface, at 91 m.sup.2 /g, as the carbon black according to DE 43 08 488. The carbon black according to DE 43 08 488 thus gives the tread composition a much lower abrasion resistance, so that it cannot be considered for use in the tread composition. On the other hand, when a standard carbon black with the same CTAB surface as the carbon black according to DE 43 08 488 is used, a steeper temperature curve can be observed. Therefore the problem above is not solved with the rubber composition disclosed in DE 43 08 488.
Kikuchi et al. U.S. Pat. No. 5,484,836 discloses rubber compositions for tires said to be low fuel consumption tires with improved heat build up and wear resistance. The carbon black used therein is disclosed to have a CTAB of 85 to 110 m.sup.2 /g based on ASTM D 3765-80 and a CDBP of greater than 105 ml/100 g. However, Kikuchi gives only relative values for tan .delta. with no absolute values.
Muraki discloses rubber compositions for tires which are intended for use with sophisticated sports cars. Here, driving stability is of prime importance at 0.degree. C. and also 60.degree. C. Therefore, Muraki tries to increase both tan .delta.-values at 0.degree. C. and at 60.degree. C. Our aim is to decrease tan .delta..sub.60 and to increase tan .delta..sub.0 at the same time (which is called the inversion effect). The aim of Muraki is to give rubber compositions for sports or racing cars with excellent gripping force on wet and dry road. Since fuel consumption is of no concern, Muraki's compositions have a high rolling resistance. Contrary to that, one of the objects of the invention is to reduce rolling resistance while at the same time retain good wet grip. Therefore, tan .delta..sub.60 must be reduced and tan .delta..sub.0 increased or at least kept constant.
The WO 94/21720 claims a rubber composition containing a special carbon black which exhibits a steeper tan .delta. temperature curve than when ordinary tire-tread carbon black is used.
To prove this allegation the applicant M. M. M. compares two rubber compositions: one containing their special carbon black Ensaco 150 and a conventional ASTM carbon black N339. In table 2 of WO 94/21720 the properties of both carbon blacks are listed.
The CTAB surface area of Ensaco 150 is 45 m.sup.2 /g and of N339 is 91 m.sup.2 /g. As explained earlier, the CTAB surface area is well known to be the most important parameter of carbon black with respect to the dynamic properties of the final rubber composition. The abrasion resistance and tan .delta. increase with increasing CTAB surface area. Comparisons made herein, therefore, comparing the rubber compositions according to the invention on the basis of the same CTAB surface area (see FIG. 2 of our application).
WO 94/21720, however, compares rubber compositions with carbon black which differ in CTAB surface area by a factor of 2.
With respect to WO 94/21720, we obtained samples of Ensaco 150 and we investigated rubber compositions containing Ensaco 150. Although the test parameters do not coincide exactly with the parameters given in our application, the different parameters are not significant in view of the magnitude of difference in the calculated tan .delta..sub.0 /tan .delta..sub.60 ratio, discussed below in more detail.
A rubber composition containing Ensaco 150 was formulated as follows:
______________________________________ Rubber Component Content (phr) ______________________________________ SBR 1712 137.5 Ensaco 150 90 ZnO RS 3 stearic acid 1 vulkanox 4010 NA 1 DPG 0.5 vulkacit NZ 1.2 sulfur 1.8 ______________________________________
The tan .delta.-values measured were:
tan .delta.=0.323 PA1 tan .delta.=0.173 PA1 tan .delta..sub.0 /tan .delta..sub.60 =1.867
According to our invention this ratio should be larger than:
______________________________________ tan .delta..sub.0 /tan .delta..sub.60 &gt; 2.76 - 6.7 .multidot. 10.sup.-3 .multidot. CTAB tan .delta..sub.0 /tan .delta..sub.60 &gt; 2.76 - 6.7 .multidot. 10.sup.-3 .multidot. 45 tan .delta..sub.0 /tan .delta..sub.60 &gt; 2.458 ______________________________________
The Ensaco 150 carbon black fails by far to supply rubber compositions with a tan .delta..sub.0 /tan .delta..sub.60 -ratio larger than 2.458. Therefore, the Ensaco 150 carbon black is not an inversion black according to the present invention.
Carbon blacks for the tire industry are now produced almost exclusively by the carbon black process (see Kirk-Other's Encyclopedia of Chemical Technology, (Third Edition), Volume 4, pages 631-666; this excerpt is entirely incorporated herein by reference). This production is based on the principle of oxidative pyrolysis, i.e., incomplete combustion of raw materials in a reactor coated with material highly fire resistant. Three zones in the reactor can be distinguished independently of the particular design of the reactor. These correspond to the three different stages of carbon black production. The zones appear in succession along the axis of the reactor and the reacting media flow through them successively.
The first zone, the "combustion zone", is essentially the combustion chamber of the reactor. The hot combustion chamber exhaust gas is produced in this zone by burning a fuel, usually hydrocarbons, with an excess of preheated combustion air or other oxygen-containing gases. Natural gas is now used predominantly as the fuel but liquid hydrocarbons such as fuel oil can also be used. The fuel is usually burned with excess oxygen. According to the book "Carbon Black", 2nd Edition, Marcel Dekker Inc., New York, 1993, page 20, the most complete possible conversion of the fuel to carbon dioxide and water in the combustion chamber is critical for optimal use of energy. The excess air promotes the complete conversion of the fuel. The fuel is usually introduced into the combustion chamber through one or more burner lances.
The "K factor" is often used as a measure to characterize the air excess. The K factor is the ratio of the volume of air needed for stoichiometric combustion of the fuel to the volume of air actually introduced for combustion. Thus a K factor of 1 indicates stoichiometric combustion. With an excess of air, the K factor is less than 1. K factors are usually between 0.3 and 0.95.
Carbon black is formed in the second zone in the furnace reactor, the "reaction zone". The carbon black yielding feedstock is injected into the stream of hot exhaust gas and mixed. The amount of hydrocarbon introduced into the reaction zone is in excess in relation to the amount of oxygen not completely consumed in the combustion zone. Therefore carbon black production normally occurs here.
Carbon black make oils of different types can be injected into the reactor. For example, it is suitable to use an axial oil injection lance or one or more radial oil lances arranged around the circumference of the reactor in a plane perpendicular to the direction of flow. A reactor can have several places with radial oil lances along the flow direction. At the heads of the oil lances there are either spray or atomizer nozzles which mix the make oil into the flow of exhaust gas.
When make oil and gaseous hydrocarbons such as methane are used simultaneously as the carbon black raw materials, the gaseous hydrocarbons can be injected into the stream of hot exhaust gas separately from the make oil through a separate set of gas lances.
In the third zone of the carbon black reactor, the "quench zone", the carbon black formation is terminated by rapid cooling of the process gas containing the carbon black. That avoids undesired later reactions. Such subsequent reactions would produce porous carbon blacks. The quenching is usually done by injecting water with suitable spray nozzles. Carbon black reactors usually have several places along the reactor where water is sprayed in for quenching, so that the residence time of the carbon black in the reaction zone can be varied. In a subsequent heat exchanger, the residual heat of the process gas is utilized to preheat the combustion air.
Many different reactor forms are known in the art. The variations described affect all three reactor zones. There are especially many designs for the reaction zone and for the arrangement of the injection lances for the carbon black raw material. They can be distributed around the circumference of the reactor or along the reactor axis. The make oil can be mixed into the exhaust gas from the hot combustion chamber better when it is divided into several separate streams. Input points distributed along the direction of flow allows staggering the oil injection with time.
The primary particle size and the specific surface of the carbon black, which is usually easy to determine, can be established by the quantity of make oil injected into the hot exhaust gas. If the volume and temperature of the exhaust gas generated in the combustion chamber are kept constant, then the quantity of make oil is solely responsible for the primary particle size and the specific surface of the carbon black. High proportions of make oil give coarser carbon blacks with lower specific surfaces than do lower proportions of make oil. The reaction temperature changes along with the change in proportion of make oil. As the make oil sprayed in reduces the temperature in the reaction, higher proportions of make oil mean lower temperatures, and vice versa. From that there follows the relation between the temperature of formation of carbon black and the specific surface of the carbon black, or the primary particle size, described on page 34 of the previously cited book, "Carbon Black".
If the make oil is divided between two different injection points spaced along the reactor axis, the residual oxygen in the exhaust gas from the combustion chamber is in excess in relation to the make oil injected at the first point upstream. Therefore, the carbon black is formed at a higher temperature there than at the subsequent injection sites. That is, more finely divided carbon black with higher specific surface forms at the first injection point than at the subsequent injection points.
Each subsequent injection of make oil causes a further decrease in temperature, producing black with larger primary particles. Thus, carbon blacks produced in this manner have a broader distribution curve for the primary particle and aggregate sizes, and behave differently after incorporation into rubber than do carbon blacks with a very narrow monomodal size distribution curve. The broader aggregate size distribution curve gives a lower loss factor in the rubber composition, i.e., lower hysteresis, so that one also speaks of low-hysteresis carbon blacks. Carbon blacks of this type, and. processes for producing them, are described in U.S. Pat. Nos. 4,988,493 and 5,124,396 (both of which are incorporated by reference in their entirety), corresponding to EP 0 315 443 and EP 0 519 988 respectively.
Thus, conventional processes are able to produce carbon blacks with broader aggregate size distribution curves by injecting the make oil at different points along the axis. When these carbon blacks are incorporated into rubber compositions they give lower rolling resistances.